1. Field of the Invention
The invention relates to portable electronic products, and more particularly, to blowers or fans particularly suitable for use in air cooling systems of portable electronic products.
2. Description of the Related Art
Axial and centrifugal fans or blowers are typically implemented in cooling systems of electronic devices to assist in cooling down the electronic devices when they become too hot. Typical fan design includes impellers that have blades spaced at equal angles relative to one another. The evenly spaced fan blades allow the impeller to be balanced. When fan blades are not spaced evenly, the impeller can have acoustic artifacts, imbalance problems, and thermal penalties. Imbalance may lead to increased vibratory stress, wear on the bearing and motor structure of the fan, and quality issues.
Typically, the noise sources of a fan are the air flow and from the motor. One of the flow-induced noise sources is the blade passage frequency (BPF) tone. The BPF and related harmonics are related to pressure disturbances produced when each fan blade passes a fixed reference point. The blade tip creates a periodic pressure wave, which creates a tone.
The major motor noise source is the pole passage frequency (PPF) tone. The PPF is the vibration and resulting pressure waves created by the poles in the motor of the fan. The BPF will usually be perceived as a tone, and can be amplified if it coincides with the PPF. The BPF and PPF tones emanate from a blower or fan, and when audible, can be annoying to the user of the product containing that blower or fan. Another source of noise is from interaction with struts or any other kind of obstruction on the fan. Thus, an adequately balanced fan with reduced noise is desired.